It is known that in the formation of a flexible printed circuit board, a layer of conductive metal foil is laminated to a layer of dielectric substrate, such as a thermosettable resin plastic, e.g., peroxide-catalyzed polyethylene. Alternatively, when the laminate is intended for use as a less-flexible, more rigid printed circuit board, in order to increase the strength of the laminate, layers of a stiffener, such as cellulose or glass cloth, mat or non-woven paper may be added to the exposed side of the dielectric substrate. Such laminates are produced by placing built up sandwiched discontinuous layers of the metallic foil, the substrate, and if desired, the stiffener, into a press or other similar device, and curing the resin layer of the laminate, e.g., with the use of peroxide, in combination with heat in a high pressure press. Clark Schwebel Fiberglass Corp., for example, sells a curable polyethylene impregnated glass cloth suitable for this purpose under the tradename RENATION.
It is also known from Lawton and Bueche, U.S. Pat. No. 2,906,678 that polyethylene can be cured in shapes such as tapes, sheets, various containers and bottles by allowing high energy irradiation obtained from high voltage accelerating apparatus to impinge on such shapes. The disclosure of the patent is incorporated herein by reference.
It has now been found that laminates can be produced continuously, without the need for a high pressure press, if a different substrate cure mechanism is selected.
It is an object of the present invention to eliminate the cumbersome and costly curing step of using a heat under pressure in combination with peroxide by curing the laminate continuously by irradiation.
It is a further object of the subject invention to provide laminates with the desired electrical properties, yet at reduced manufacturing costs.